Much of the material printed with drying oil-based inks is used in applications which require that the surface over the printed image have a high gloss. The conventional approach to achieving such a finish is through lamination of a clear plastic or cellulosic film over the printed sheet after the inks have cured, or by the off-line application of a laquer or varnish, also after the inks have cured. The advent of commercially available, radiation-curable coatings has made it possible to apply a specially formulated, low-viscosity varnish to the surface of the print in-line with the printing press while the ink is in the uncured state. This varnish can be very efficiently cured right after application to yield a smooth, tack-free surface which protects the ink in the stack or roll of printed material while the ink cures gradually by oxidation. The use of a radiation-curable varnish in this way also allows the elimination of the anti-set-off spray powder often used in the sheet-feed, lithographic process; this powder is difficult to keep out of the pressroom environment and gives a rough, gritting feeling to the print surface.
There have been many attempts by those skilled in the art to formulate a radiation-curable varnish which can be applied in a thin film of a thickness 0.0003 inch or less, that will flow out to a smooth, ripple-free surface before curing and that will cure at practical line speeds of from 300 to 400 feet per minute or greater to a reasonably flexible, adherent film having a high gloss.
The main difficulty that has been encountered in trying to achieve this result has been the reduction in gloss level that occurs over heavy films of some inks as they cure under the already-cured varnish film. The inks appear to increase in surface roughness as they cure, causing the overlying varnish film, which is deformable in most practical cases, to also take on a surface roughness. This problem is exacerbated by the tendency of all-organic acrylates to diffuse into and mix with the wet ink film before they are cured. Furthermore, it is believed that certain lower molecular weight components of ink and the volatile, oxygenated products of drying oil oxidation can diffuse into the cured, all-organic acrylate varnish film resulting in uneven deformation, such as micro-wrinkling, of the varnish film. All of these proposed mechanisms work to produce an uneven and consequently lower-gloss varnish surface.
A radiation-curable coating which can be applied in thin films over oil-based ink prints and cures to a high gloss finish would be highly desirable.